The present invention relates to a phase angle control circuit for motors having a command stage and a control stage for driving thyristors of rectifier bridges, with one current transformer being provided for determining the actual value of the motor current with a single-phase a-c current supply, and two current transformers with a three-phase a-c current supply, which current transformers, in addition, are utilized for determining the current zero-axis crossing before motor current reversal and for determining the duration of the current rest time.
As is known, in order to be able to control d-c motors, knowing the actual value of the motor current is required, since automatic current regulation is subordinate to the control. With d-c motors, which are supplied from an a-c network with current obtained through phase angle control, it is customary to provide current transformers, which are connected on the primary side to the supply a-c network and from the secondary voltage of which, with the aid of a rectifier circuit, a value is obtained which represents the actual current value.
For motors which are driven in both rotational directions, two controlled rectifier bridges are customarily provided, of which, however, only one can be in operation at any given time. One must absolutely and at all cost prevent that one rectifier bridge is being operated before the other has completely stopped operating, since, otherwise, through the thyristors of different rectifier bridges, short-circuits come about which can destroy the rectifier bridges.
For the control, therefore, before initiating current direction reversal causing reversal of the direction of rotation, the exact determination of the point of zero current in the thyristors of the rectifier bridge in operation up to this point, is required for the control, before the thyristors of the other rectifier bridge, inhibited until then, are released, because a thyristor's ability to block current is only given at zero current.
It is known, to detect the actual current value required for the purpose of current regulation with the aid of current transformers, since, in general only the absolute value of the current is of interest for the regulation and the regulator is designed only for evaluating d-c current, that the secondary voltage of the current transformer is normally rectified.
If, however, a current transformer is already available for the purpose of determining the actual current value, then it is convenient to utilize this current transformer also for determining the current zero-axis crossing. For this purpose, however, the d-c voltage tapped after the rectifiers cannot be used directly for the following reason:
Before the current direction is reversed, the current flowing until then, as already mentioned, must be reduced to zero, before the current flowing in the opposite direction can become effective. A phase-controlled current, however, degenerates during zero-axis crossing into individual current pulses (interrupted current) which become increasingly smaller. Between these pulses the current is actually zero, so that during such a current rest time between two current pulses, current direction reversal can be initiated. One is now faced with the task of recognizing the beginning of such a current rest time through the preceding period in which the current becomes zero, in order to initiate current direction reversal. In this connection, the following fact is of importance:
From a preceding current pulse, the current transformer is magnetized to such an extent, that after the current reaches zero, a residual magnetic energy remains, the reduction of which acts on the secondary side (wit respect to the current direction up to this point) as a negative voltage pulse. If one were, as it is known, to obtain the actual current value at any given instance through rectification, then these negative voltage pulses would also be rectified and thus, simulate a current flow which actually does not exist.
Such voltage obtained by rectification would be unsuitable for recognizing a current zero-axis crossing, since these negative voltage pulses due to demagnetization, would also be represented positive and, since they fade asymptotically, would indicate an erroneous point of current zero-axis crossing. Thus, the precise time of the current zero-axis crossing could not be detected exactly, which would impair both the safety of operation and the dynamics.
FIG. 3a shows the course of the supply a-c voltage. In FIG. 3b, the voltage time area gained through phase-drive and after full-wave rectification for the motor supply is shown. FIG. 3c shows current pulses obtained from a single-phase current transformer, at the ends of which the voltage peaks occur, directed opposite to the direction of the pulse voltage due to the reduction of the residual magnetic energy. If these current pulses were rectified with a rectifier circuit, then the series of current pulses shown in FIG. 3e would result, the end voltage peaks of which would show the same polarity as the current pulses due to the rectification which is indicated with hatched lines. These rectified end voltage peaks, however, would make exact recognition of the current zero-axis crossing impossible and thus falsify the points of zero-axis crossing.